Electronic ballast circuits use filament drive circuits to provide a heating voltage to the filaments of a gas-discharge lamp and to ensure that the filaments are properly heated during the operation of the lamp. A filament drive circuit may generate a pulsed heating signal to maintain the filaments at the appropriate temperature. Some gas-discharge lamps require that the heating effect of the pulsed heating signal be adjusted in accordance with the lamp current. This is particularly true if the gas-discharge lamp is connected to a dimmable ballast circuit that adjusts the lamp current in accordance with a desired dimming level.
Ballast circuits may have an inverter that operates to convert a DC voltage into an AC voltage at the appropriate frequency for operating the gas-discharge lamp. To do this, inverter switch devices receive the DC voltage and are switched on and off at a switching frequency to generate a periodic signal. A control circuit coordinates the switching of the inverter switch devices using clock signals. This periodic signal is then filtered through a resonant circuit to create the appropriate AC voltage for powering the gas-discharge lamp. The problem with prior art filament drive circuits is that they either interfere with the operation of the inverter or they require a complicated coordination scheme with the clock signals to adjust the heating effect of the pulsed heating signal.
For example, prior art filament drive circuits may couple a filament drive resonant tank in parallel with the resonant circuit in the inverter. Adjusting the resonant frequency of the filament drive resonant tank adjusts the heating effect of the pulsed heating signal. Unfortunately, this also has the effect of adjusting the resonant frequency of the inverter's resonant circuit. Complicated and expensive circuitry is required to achieve the desired heating effect while maintaining the desired dimming level of the lamp.
Other prior art filament drive circuits use a clock signal to coordinate the generation of the pulses. Unfortunately, these circuits require a processor to determine the number of clock cycles associated with a desired pulse width for the pulsed heating signal and to count clock cycles during the generation of a pulse. A processor then implements a complicated digital counting scheme to time the start and end of the pulses which reduces the reliability of the circuit and increases its cost.
What is needed is a filament drive circuit for an electronic ballast that does not significantly affect the resonant circuit of the inverter and that does not require complicated counting schemes to provide the appropriate heating effect to the lamp filaments.